Radiosynthesis of acid chlorides

ABSTRACT

Radiolabelled acid chlorides may be synthesised by reacting a radiolabelled carboxylic acid with a solid-phase supported chlorinating agent.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of radiosynthetic chemistry. In particular, the invention relates to a method for the radiosynthesis of a radiolabelled acid chloride intermediate, which can be used in the preparation of radiolabelled products such as radiolabelled amides, amines and esters.

DESCRIPTION OF RELATED ART

The established radiosynthesis of a radiolabelled acid chloride intermediate (illustrated as R—COCl below) from CO₂ is via the route:

(i) CO₂+Grignard reagent→R—COOH (ii) R—COOH+chlorinating agent→R—COCl wherein R can be aromatic, short or long chain aliphatic. The chlorinating agents used, such as thionyl chloride and oxalyl chloride, need to be removed from the reaction mixture once the radiolabelled acid chloride intermediate has been formed. This is because they are very reactive and can cause chemical side reactions in any subsequent reaction in which the radiolabelled acid chloride intermediate is used. For example, when the subsequent reaction is between the radiolabelled acid chloride intermediate and an amine, the reaction between any remaining chlorinating agent and amine will result in a complex mixture and low yield of radiolabelled amide product. In addition, chlorinating agents are typically toxic and as such unsuitable where a compound is being prepared for human administration. When the radiolabelled acid chloride intermediate has a relatively low boiling point compared with the chlorinating agent, separation by distillation may be conveniently carried out [Luthra et al 1990 Appl. Radiat. Isot., 41(5) pp 471-476]. However, when the radiolabelled acid chloride intermediate has a relatively high boiling point compared with the chlorinating agent, separation by distillation from the chlorinating agent is more difficult and there is a chance that traces of chlorinating agent will remain in the final product.

To resolve the issue of separating radiolabelled acid chloride intermediates of relatively high boiling point, Luthra et al [1990 Appl. Radiat. Isot., 41(5) pp 471-476] heated the tubing between the reaction vessels and succeeded in isolating [¹¹C]-cyclobutanecarbonyl chloride (boiling point 140° C.) from the reaction mixture in a radiochemical yield of between 35 and 60%.

In another attempt to resolve the separation problems, e.g., McCarron et al [1996, J. Label Comp. Radiopharms, 38(10), pp 941-953] used immobilised Grignard reagent in the radiosynthesis of the relatively involatile acid chloride intermediate, [carbonyl-¹¹C]cyclohexanecarbonyl. This meant that only small quantities of all reagents were required, thereby simplifying the purification process.

In addition to problems with removal of chlorinating agents, any separation step adds time to the radiosynthesis, leading to decay of the radioisotope, which would be desirably avoided. This is especially significant where the half-life of the radioisotope is relatively short, e.g. in the case of ¹⁵O (half-life=2.07 minutes), ¹³N (half-life=9.965 minutes), ¹¹C (half-life=20.4 minutes) and ¹⁸F (half-life=109.7 minutes). Decay results in a reduction of the specific activity of the radiolabelled product overtime. Specific activity is particularly important when the radiolabelled products which are radiopharmaceuticals, as the non-radioactive cold carrier competes with the radiolabelled product. Time is therefore a reaction parameter of equal importance to chemical yield for such short-lived radiolabelled products. The relationship between time and concentration of reactants with respect to reaction kinetics is described in the literature [Långström et al, 1981, J. Radionnal. Chem. 64 pp 273-80].

A method for the radiosynthesis of radiolabelled acid chloride intermediates that eliminated the need for a separation step would therefore be advantageous.

SUMMARY OF THE INVENTION

The difficulties presented by the prior art methods have been surmounted by a method which uses a solid phase-supported chlorinating agent for the conversion of a radioactive carboxylic acid to a radioactive acid chloride intermediate. As the chlorinating agent is a solid reagent, there is no chlorinating agent in the final acid chloride solution. This avoids the necessity for an additional separation step such that there is no issue with residual non-radioactive chlorinating agent in the product. Furthermore, the method can be completed in less time than the prior art methods.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect the present invention relates to a method for the preparation of a radiolabelled acid chloride compound of formula (I):

R—COCl  (I)

wherein R is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₁₀ cycloalkyl, C₃-C₁₀ heterocyclyl, C₄-C₁₀ aryl or C₄-C₁₀ heteroaryl, any of which may optionally be substituted with NO₂, —O(C₁-C₆ alkyl), C₃-C₁₀ cycloalkyl or C₄-C₁₀ aryl; the method comprising reacting a radiolabelled carboxylic acid of formula (II):

R—COOH  (II)

wherein R is as defined for formula (I); with a solid-phase supported chlorinating agent in the presence of a base.

In the context of the present invention, the term “C₁-C₂₀ alkyl” refers to a fully saturated straight or branched hydrocarbon chain containing from 1 to 20 carbon atoms. Examples include methyl, ethyl, isopropyl, t-butyl and n-decyl. The term “C₂-C₂₀ alkenyl” refers to a straight or branched hydrocarbon chain containing from 2 to 20 carbon atoms and containing one or more C═C bonds. Examples include ethenyl, propenyl and 3-decenyl.

The term “C₃-C₁₀ cycloalkyl” refers to a cyclic fully saturated hydrocarbon group having from 3 to 6 ring carbon atoms. The cycloalkyl group may comprise either a single ring or a fused system. Examples include cyclopropyl, and cyclohexyl.

The term “C₃-C₁₀ heterocyclyl” refers to a C₃-C₁₀ cycloalkyl group as defined above but in which one of the ring carbon atoms is replaced by —S—, —O— or —NH—. Examples include tetrahydrofuranyl, morpholinyl and piperidyl.

The term “C₄-C₁₀ aryl” refers to a cyclic hydrocarbon having aromatic character and containing from 4 to 10 ring atoms. The aryl group may be a single ring, a fused aromatic system or an aromatic ring fused to a cycloalkyl or heterocyclic ring. Examples of aryl groups include phenyl, naphthyl and indolinyl,

The term “C₄-C₁₀ heteroaryl” refers to a C₄-C₁₀ aryl group as defined above but in which one of the ring carbon atoms is replaced by —S—, —O— or —NH—. Examples include pyridyl, quinolyl and indolyl.

The term “halo” refers to fluoro, chloro, bromo or iodo.

Where a compound is defined as “radiolabelled” in the present invention, this signifies that the compound comprises a radioactive isotope. The radioactive isotope may be an inherent part of the compound structure, or may alternatively be chemically attached to the compound via a suitable chemical group which optionally comprises a linker.

In the context of the present invention, a compound which is “solid-phase supported” is chemically attached to any suitable solid-phase support which is insoluble in the solvents to be used in the radiosynthesis.

Some of the solid phase supported chlorinating agents which are useful in the present invention comprise a chlorinating agent connected by a linker to a solid support. Examples of suitable solid supports include polymers such as polystyrene (which may be block grafted, for example with polyethylene glycol), polyacrylamide, or polypropylene or glass or silicon coated with such a polymer. The solid support may also be in the form of small discrete particles such as beads or pins, or a coating on the inner surface of a cartridge or on a microfabricated vessel.

In this type of structure, the solid-phase supported chlorinating agent may be, for example, a solid-phase supported acid chloride, or a solid-phase supported analogue of either thionyl chloride or oxalyl chloride. The chemical attachment of the chlorinating agent to the solid support must be such that its activity as a chlorinating agent is maintained.

Preferably, the solid-phase supported chlorinating agent is a solid-phase supported acyl chloride of Formula III:

[solid support-linker—COCl  (III)

wherein the linker is a polyethylene glycol linker or comprises up to four groups selected from: C₄-C₁₀ aryl or C₄-C₁₀ heteroaryl groups; (CH₂)_(x) groups where x is an integer from 1 to 20; (CH₂)_(y)—O—(CH₂)_(z) groups where y and z are integers from 1 to 20; or combinations thereof.

In the linker groups, any CH₂ group may be replaced by —O—, —S—, —SO₂— or —NH₂— and the linker groups may be substituted with one or more substituents chosen from OH, halo, amino, nitro and C₁-C₆ alkoxy.

A preferred solid-phase supported acyl chloride is of Formula IIIa:

The above solid-phase supported acyl chloride can be prepared from commercially available polystyrene-supported carboxylic acid (Novabiochem Cat. No. 01-64-0111), by methods reported in the literature [Leznoff et al 1977 Can. J. Chem. 55(19) pp 3351-3355; Meyers et al 1995 Molecular Diversity 1, pp 13-]. These methods describe the transformation of polystyrene-supported carboxylic acid to polystyrene-supported acyl chloride by treatment with oxalyl chloride or thionyl chloride (see FIG. 1).

Alternatively, the solid phase supported chlorinating agent may be an integral part of a polymer. For example, polymerisation of a monomer containing an acid anhydride group gives a polymer which also contains the anhydride group on every unit. Each of the anhydride groups may then be converted to two acid chloride groups by known processes, for example by heating the polymer in the presence of a chlorinating agent such as phosphorus pentachloride [Hesse et al, 1982, Liebigs Ann. Chem., 11, 2079-2086], thionyl chloride [Cantrell et al, 1977, J. Org. Chem., 42, 3562-3567] or zinc chloride [Johnson et al, 1982, J. Am. Chem. Soc., 104, 2190-2198].

An example of this process for producing solid phase supported chlorinating agents of this type include the polymerization of anhydride monomers of type (1) by radical initiated polymerisation or monomers of type (2) by transition metal catalysed ring-opening metathesis polymerisation (ROMP)

followed by their conversion to the equivalent poly(acid anhydrides).

In a similar manner an ester-containing monomer can be polymerised and the ester groups on the polymer converted to acid chloride groups by reaction with a base such as potassium hydroxide followed by reaction with a chlorinating agent such as thionyl chloride [Hagemann et al, 1997, Synth. Commun., 27, 2539-2546]

An example of this is the radical initiated polymerisation of monomers of type (3) followed by activation of the ester to the corresponding solid phase supported acid chloride.

The reaction of the compound of formula (II) with the solid-phase supported chlorinating agent is carried out in the presence of a base.

Suitable bases include pyridine derivatives, for example alkyl pyridines such as di-^(t)butylpyridine.

Alternatively, the base may, like the chlorinating agent, be immobilised on a solid support. This may be achieved, for example by polymerization of a pyridine-substituted alkene monomer such as a vinyl-di-^(t)butylpyridine monomer:

In another variation, the base may be immobilised in the same polymer chain as the acid chloride chlorinating agent. This may be achieved by polymerising a monomer which contains both an ester or anhydride functional group and a pyridine functional group. The anhydride or ester groups in the resulting polymer can be converted to acid chloride functional groups as described above. An example of this is shown below.

Conversion of the radiolabelled carboxylic acid of formula (II) to the radiolabelled acid chloride compound of formula (I) is preferably conducted by contacting the solid-phase supported chlorinating agent with a solution of the carboxylic acid.

It is greatly preferred to carry out the reaction by passing the solution of the compound of formula (II) through a column packed with a resin containing the solid phase supported chlorinating agent.

The solution may contain a base or, alternatively, a base may immobilised on a solid support as described above.

In a preferred embodiment of the invention, the radiolabelled acid chloride intermediate is radiolabelled with a radioactive imaging moiety. The term “radioactive imaging moiety” in the context of the present invention is taken to mean a radioactive isotope that may be detected external to the human body in a non-invasive manner following its administration in vivo. Examples of suitable radioactive imaging moieties of the present invention are:

-   -   (i) a positron-emitting radioactive non-metal selected from ¹¹C,         ¹³N, ¹⁷F, ¹⁸F, ⁷⁵Br, ⁷⁶Br and ¹²⁴I; or     -   (ii) a gamma-emitting radioactive halogen selected from ¹²³I,         ¹²⁵I, ¹³¹I or ⁷⁷Br.

A preferred radioactive imaging moiety of the invention is a positron-emitting radioactive non-metal selected from ¹¹C, ¹³N and ¹⁸F. These radioactive imaging moieties are particularly well suited for use as radiopharmaceuticals due to their physical and biochemical characteristics. In particular, the relatively short half-lives of these radioactive imaging moieties means that radiation exposure to a patient undergoing a PET scan is minimised. It follows that shortening the time taken to produce radiopharmaceuticals comprising such radioactive imaging moieties is particularly advantageous.

When the radioactive imaging moiety of the invention is ¹¹C, it is preferably an inherent part of the radiolabelled acid chloride intermediate. Thus, the radiolabelled carboxylic acid of formula (II) is preferably of the formula R-¹¹COOH and the radiolabelled acid chloride compound of formula (I) is preferably of the formula R-¹¹COCl.

¹¹C may be produced by proton bombardment of natural nitrogen through the ¹⁴N(p,α)¹¹C nuclear reaction using a target gas mixture of 2% oxygen in nitrogen to produce radioactive carbon dioxide (¹¹CO₂). The ¹¹CO₂ may then be reacted with Grignard reagent to give R-¹¹COOH and these steps may also form part of the process of the invention.

Next, a solution comprising R-¹¹COOH and any remaining Grignard reagent is passed through a column containing the solid-supported acyl chloride of Formula (III). This is illustrated in FIG. 1, where the acyl chloride is immobilized on a polystyrene support. An exchange reaction occurs between the excess solid-supported acyl chloride and the compound of formula (II), R-¹¹COON to produce a solution of the compound of formula (I), R-¹¹COCl.

The compound of formula (I) is useful as an intermediate in the preparation of radiolabelled products.

Therefore, the method of the invention may include the additional step of preparing a radiolabelled amide, amine or ester by reacting a radiolabelled acid chloride compound of formula (I) as described above with an appropriate reagent.

The reaction schemes below show reactions of ¹¹C-labelled acid chlorides of formula (I) but they are also applicable to reaction schemes below are also illustrative of the equivalent non-radioactive reactions.

Radiolabelled amides may be obtained by the following reaction:

R-¹¹COCl+R′R″—NH→R-¹¹CON—R′R″

wherein R is as defined previously; and R′ and R″ are each independently hydrogen, C₁-C₂₀ alkyl or C₂-C₂₀ alkenyl, wherein alkyl or alkenyl groups may be substituted with one or more substitutents chosen from OH, halo, amino, nitro, C₁-C₆ alkoxy, C₃-C₁₀ cycloalkyl, C₃-C₁₀ heterocyclyl, C₄-C₁₀ aryl or C₄-C₁₀ heteroaryl, and wherein one or more CH₂ groups of the alkyl or alkenyl chain may be replaced by an —S—, —O— or —NH— group; or R′ and R″ may be taken together with the N to which they are attached to form an aliphatic, aromatic or partially aromatic N-containing heterocycle having 5 to 30 ring atoms and up to 6 rings, which may contain one or more additional heteroatoms selected from N, O and S or in which a ring CH₂ may be replaced by C═0 and which may be substituted with one or more substituents selected from C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, OH, halo, amino, nitro, C₁-C₆ alkoxy, C₃-C₁₀ cycloalkyl, C₃-C₁₀ heterocyclyl, C₄-C₁₀ aryl or C₄-C₁₀ heteroaryl.

The corresponding radiolabelled amines can be formed by the reduction of the radiolabelled amides by lithium aluminum hydride (LiAlH₄) as follows:

wherein R, R′ and R″ are as defined previously.

Certain radiolabelled amines can be prepared directly from the R-¹¹COCl, for example [¹¹C]-cyclopentyltheophylline [Yorke et al 1994 J. Label. Compd. Radiopharm.35 pp 262-3]:

Radiolabelled esters may be obtained by the following reaction:

R-¹¹COCl+R′″OH→R—CO₂—R′″

wherein R is as defined previously and R′″ is a group as defined previously for R′ and R″.

Preferably, the radiolabelled product is a radiopharmaceutical, preferred examples [including references to prior art methods of synthesis] of which are given in Table 1.

TABLE 1 Structures of preferred radiopharmaceuticals of the invention Radiopharmaceutical Structure Prior art reference [carbonyl-¹¹C]-WAY- 100635

McCarron et al 1996 J. Label. Compd. Radiopharm. 38 pp 941-53 [¹¹C]-propyl- norapomorphine

Hwang et al 2000 Nucl Med Biol. 27 (6) pp 533-9 [¹¹C]-diprenorphine

Luthra et al 1985 J. Chem. Soc. Comm. 70 pp 1423-5 [¹¹C]-buprenorphine

Luthra et al 1987 Appl. Radiat. Isot. 38 pp 65-6 [¹¹C]- cyclopentyltheophylline

Yorke et al 1994 J. Label. Compd. Radiopharm. 35 pp 262-3

In a further aspect the present invention relates to a kit for carrying out the method of the invention comprising:

-   -   (i) a first cartridge containing a solid-phase supported         chlorinating agent; and,     -   (ii) a vial containing a solution suitable for dissolving the         radiolabelled carboxylic acid of formula (II), or components         which can be reconstituted to form such a solution.

The dissolved radiolabelled carboxylic acid of formula (II) is passed through the cartridge wherein it reacts with the solid-phase supported chlorinating agent to form the radiolabelled acid chloride of formula (I).

The solution for dissolving the radiolabelled carboxylic acid of formula (II) may also contain a base, for example a pyridine base such as di-^(t)butoxypyridine. Alternatively, the first cartridge may contain a solid-phase supported base, which may be a polymer as defined above.

Suitable solid-supported chlorinating agents and bases are as defined above in relation to the first aspect of the invention.

In another aspect, the present invention relates to a kit for the radiosynthesis of a radiolabelled product comprising the kit for carrying out the method of the invention and:

-   -   i) a second cartridge containing a solid-phase supported         non-radioactive precursor of the radiolabelled product.

In use, the radiolabelled acid chloride of formula I is passed through the second cartridge where it reacts with the solid-phase supported non-radioactive precursor of the radiolabelled product to form the radiolabelled product.

Non-radioactive precursors of amines, amides and esters are described above.

BRIEF DESCRIPTION OF THE EXAMPLES

Example 1 describes how a polystyrene-supported acyl chloride is obtained.

Example 2 describes the radiosynthesis of a [¹¹C]-acid chloride.

Example 3 describes the radiosynthesis of a [¹¹C]-amide.

Example 4 describes the radiosynthesis of a [¹¹C]-amine.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the reactions involved in the preparation of the polystyrene-supported acyl chloride and also the radiosynthesis of the radiolabelled acid chloride intermediate.

EXAMPLES Example 1 Preparation of Polystyrene-Supported Acyl Chloride

Polystyrene-supported carboxylic acid [Novabiochem Cat. No. 01-64-0111] is converted to polystyrene-supported acyl chloride by treatment with either oxalyl chloride or thionyl chloride by methods disclosed in the prior art [Leznoff et al 1977 Can. J. Chem. 55 p 3351-, Meyers et al 1995 Molecular Diversity 1 p 13]. FIG. 1 illustrates this reaction.

Example 2 Radiosynthesis of an [¹¹C]-Acid Chloride

A solution of [¹¹C]-labelled carboxylic acid and Grignard reagent is passed through a column containing polystyrene-supported acyl chloride. An exchange reaction occurs between the excess polystyrene-supported acyl chloride and the no carrier added carboxylic acid to produce a solution of [¹¹C]-labelled acid chloride. FIG. 1 illustrates this reaction.

Example 3 Radiosynthesis of a [¹¹C]-Amide

The solution of [¹¹C]-labelled acid chloride produced in Example 2 is reacted with a secondary amine to obtain the respective [¹¹C]-labelled amide product.

Example 4 Radiosynthesis of a [¹¹C]-Amine

The [¹¹C]-labelled amide product obtained in Example 3 is reduced in the presence of lithium aluminum hydride (LiAlH₄) to obtain the respective [¹¹C]-labelled amine.

Example 5 Radiosynthesis of a [¹¹C]-Ester

The [¹¹C]-labelled acid chloride of Example 2 is reacted with an alcohol in the presence of pyridine or NaOH to obtain the respective [¹¹C]-labelled ester. 

1-24. (canceled)
 25. A kit for carrying out a method for the preparation of a radiolabelled acid chloride compound of formula (I): R—COCl  (I) wherein R is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₁₀ cycloalkyl, C₃-C₁₀ heterocyclyl, C₄-C₁₀ aryl or C₄-C₁₀ heteroaryl, any of which may optionally be substituted with NO₂, —O(C₁-C₆ alkyl), C₃-C₁₀ cycloalkyl or C₄-C₁₀ aryl; the method comprising reacting a radiolabelled carboxylic acid of formula (II): R—COOH  (II) wherein R is as defined for formula (I); with a solid-phase supported chlorinating agent in the presence of a base, the kit comprising: (i) a first cartridge containing a solid-phase supported chlorinating agent; and, (ii) a vial containing a solution suitable for dissolving the radiolabelled carboxylic acid of formula (II), or components which can be reconstituted to form such a solution.
 26. A kit as claimed in claim 25, wherein the solid-phase supported chlorinating agent comprises a chlorinating agent connected by a linker to a solid support.
 27. A kit as claimed in claim 25, wherein the solution for dissolving the radiolabelled carboxylic acid of formula (II) also contains a base.
 28. A kit as claimed in any one of claims 25 wherein the first cartridge contains a solid-phase supported base is immobilised on a solid support.
 29. A kit for the radiosynthesis of a radiolabelled product comprising a kit as claimed in claim 25 and: (iii) a second cartridge containing a solid-phase supported non-radioactive precursor of the radiolabelled product. 